In the contemporary environment of the present time, it is becoming increasingly more important to recycle plastics, including particularly poly(ethylene terephthalate) (hereinafter referred to as "PET") which is used in volumes of billions of pounds per year, mainly to package carbonated drinks. Concern for the environment has led to a demand for packaging materials that are at least partially composed of postconsumer materials. As a result, postconsumer polyester materials are now being widely recycled. For instance, carbonated beverage bottles made of PET may be recycled into new PET beverage bottles.
Due to problems with overloaded landfills, several states have passed laws requiring certain percentages of new PET soft drink bottles to be made from clean postconsumer polyester material generated from previously used PET beverage bottles. In the past few years the number of bottle deposits and curbside recycling programs has increased tremendously and such activities are providing a source of postconsumer packaging materials including bottles for carbonated beverage made from PET. The collection of PET bottles and food trays through recycling programs has resulted in a source of postconsumer PET which has been used for a number of applications.
Generally speaking, these known applications have involved the conversion and fabrication of the postconsumer PET into materials of lower value. Examples of such applications are polyols for unsaturated polyesters or polyurethanes, fiberfill, carpet fibers, and strapping. Recycled PET is also blended with other materials such as poly(butylene terephthalate) or polycarbonate, for automotive as well as other engineering applications.
These known procedures all require a myriad of expensive processes to accomplish the desired blending of old and new materials, and all have shortcomings which limit their adaptability for use in the production of the high grade materials needed for packaging ingestible products. Known procedures for depolymerizing PET for reuse in the manufacture of new PET are described, for example, in U.S. Pat. Nos. 3,703,488, 3,884,850 and 5,223,544.
Presently, it has become commercially desirable to use previously used, particularly postconsumer PET in the synthesis of new PET for making carbonated beverage bottles. Several chemical treatment techniques are known for facilitating the regeneration and recycling of previously used polyester material. Such techniques are employed to depolymerize polyester material to be recycled, whereby the polyester material is reduced to monomeric and/or oligomeric components. The monomeric and/or oligomeric components may then be repolymerized to produce recycled polyester material. Such techniques may be used to facilitate the recycling of PET; however, it should be apparent to the routineer in the related arts that these same techniques are applicable to other polyester materials which are desirably to be recycled.
One known technique is to subject PET to methanolysis. In accordance with the methanolysis approach, PET is reacted with methanol to produce dimethyl terephthalate (DMT) and ethylene glycol (EG). The DMT and EG may be readily purified and thereafter used to produce PET containing recycled polyester material. However, most conventional commercial PET production facilities throughout the world are designed to use either terephthalic acid (TPA) or DMT, but not both, as the monomeric raw material. Thus, additional processing is generally required to convert the DMT into the TPA needed as a raw material for many such facilities.
Another known technique is hydrolysis, whereby PET is reacted with water to depolymerize the PET into TPA and EG. However, it is known that certain types of contaminants generally present in recycled PET are very difficult and expensive to remove from TPA. Moreover, for those facilities designed to use DMT as a raw material, the TPA must be converted into DMT.
Glycolysis may also be used for depolymerizing PET. Glycolysis occurs when PET is reacted with EG, thus producing bis-(2-hydroxyethyl) terephthalate (BHET) and/or its oligomers. Glycolysis has some significant advantages over either methanolysis or hydrolysis, primarily because BHET may be used as a raw material for either a DMT-based or a TPA-based PET production process without major modification of the production facility. Another significant advantage provided by the glycolysis technique is that the removal of glycol from the depolymerization solvent is not necessary. In this connection it is to be noted that the co-produced glycol must be quickly separated from the water in the hydrolysis process and from the methanol in the methanolysis process to avoid immediate repolymerization.
The glycolysis approach is not without its own problems. These problems are discussed by Richard et al., for example, in their article entitled Incorporating Postconsumer Recycled Poly(ethylene terephthalate) which appeared in ACS Symposium Series, 513, 196 (1992).
A shortcoming of the previous glycolysis technique, as well as of all of the other known techniques described above, however, is that additional processing steps, as well as the concomitant additional capital expenditures, are required.
Previously known glycolysis processes include the independent complete glycolysis of postconsumer PET and the subsequent addition of some portion of the glycolysis product to a polycondensation process. Such processes require more than normal pressure and a large excess of ethylene glycol. These requirements reduce the reactor efficiency by decreasing the potential production capacity of the reactor. Such a process is described in U.S. Pat. No. 5,223,544. Thus, it has previously been known that polyester molecules may be depolymerized by glycolysis. However, in the past it has been found that high temperatures and large excesses of EG are required to solubilize the polyester molecule so that it can then be broken down into its constituent parts such as BHET and oligomers thereof. The high temperatures and excessive EG results in the production of large quantities of diethylene glycol as a by-product. The diethylene glycol thus produced can not be readily removed from the BHET and so, if the BHET is then used to produce regenerated PET, the resultant PET product has a diethylene glycol content that is excessive causing the polymer to be unacceptable for many commercial uses.
Other known processes involving glycolysis require the retention in the reactor of a heel of BHET oligomer having a degree of polymerization greater than 10 at the end of a reaction run in order to solubilize the postconsumer PET because the latter is insoluble in most solvents. These procedures are described in U.S. Pat. No. 4,609,680.
Thus, there clearly remains a need in the art for a glycolysis process that can efficiently handle previously used PET in the manufacture of new packaging grade PET.